1. Field of the Invention
This invention relates to the field of wireless communication, and more specifically to antennas for radiating and receiving both circular polarized (CP) and linear polarized electromagnetic signals, for example signals that are used in satellite communication systems.
2. Description of the Related Art
Mobile satellite communication systems create a need for low profile and compact antennas. For example, satellite radio systems include both satellite transmitters and terrestrial or land-based transmitters, and mobile antennas that are used in these satellite radio systems are required to receive both satellite transmitted signals and terrestrial transmitted signals. In addition, this signal redundancy must be designed into the system so that there will be few geographic regions providing gaps in coverage across the country.
Terrestrial signals are much stronger than satellite signals. However, in order to be economical, terrestrial transmitters are usually placed around large metropolitan centers, since it is cost prohibitive to place terrestrial transmitters in relatively unpopulated regions of the country. However, satellite signals are provided virtually everywhere, and such signals are required for regions of the country that do not receive terrestrial transmitted signals.
A low profile satellite antenna is desired for automotive applications due to obstacles that such an antenna may encounter, for example soccer balls, rollers that are within a car wash, and items that may be temporarily mounted on the roof of the automobile.
A low profile automobile antenna is also desired because such an antenna can be easily factory-installed, and the antenna runs less risk of being damaged before arriving at an auto dealership. An additional reason favoring low profile automobile antennas is their relatively pleasing appearance, and the fact that low profile antennas do not generally suppress visibility.
In the example of a satellite radio system, it is a technical challenge to fit desired antenna functions within a single, low profile and compact antenna assembly for mounting on the top of an automobile.
A low profile CP patch antenna is usually not adequate to serve as a satellite antenna, unless the automobile is located relatively close to the equator. The directivity of a patch antenna that is located over a large ground plane is usually over 5 dB when the antenna points directly up.
From the vantage point of geographic areas within the United States, geo-stationary satellites are located predominantly between 20 and 60 degrees off of the southern horizon. Hence, signals that are received from a geo-stationary satellite using a CP patch antenna are weak signals.
A solution to providing a satellite antenna is a quadrifilar helix antenna. FIG. 1 shows a standard-technology antenna 10 having both a quadrifilar helix 11 and a concentrically-located monopole 12. Quadrifilar helix antenna 11, when fed in quadrature, generates an omni CP depressed cardioid pattern, which is an omni pattern with a moderate (i.e. a few dB) dip in gain at zenith. Monopole antenna 12 generates a linear omni pattern. Coupling between CP quadrifilar helix antenna 11 and monopole antenna 12 can be reduced by placing the monopole antenna 12 in the geometric center of helix antenna 11.
Quadrifilar helixes 11 as shown in FIG. 1 are typically over two wavelengths tall, this height being required in order to generate a depressed cardioid pattern. As can be seen from FIG. 1, such an antenna does not have a low profile, and such an antenna is not physically compact.
A lower profile standard-technology antenna is a crossed dipole antenna, wherein the dipole must be xe2x85x9c wavelength or more above a ground plane in order to generate a depressed cardioid pattern. If the dipoles of such an antenna are closer to the ground plane, directivity of the antenna is too large, and the antenna pattern is similar to that of the CP patch antenna described above.
FIG. 2 shows a standard-technology droopy crossed dipole antenna 13 having four combined monopoles 14 that are fed 90 degrees out of phase in order to generate CP radiation. The four meanderline monopoles 14 of FIG. 2 are fed in phase and they are combined underneath the antenna with a feed network (not shown), to thus provide a single linear monopole pattern. Monopoles 14 of FIG. 2 can be straight wires, they can be planar inverted-F antennas (PIFAs), or they can be top loaded monopoles, all of which create the same radiation.
Coupling between the crossed dipoles 15 of FIG. 2, and feed to monopoles 14, is ideally zero because coupling to each of the four monopoles 14 is in quadrature, and this coupling cancels at the input to the antenna""s feed network. However, the xe2x85x9c wavelength height that is required in antenna 13 does not provide a low profile antenna for mounting on the top of an automobile.
Low profile antennas that generate a conical CP pattern and that have a deep null at zenith, instead of a depressed cardioid pattern, are available. FIG. 3 shows a standard-technology ring antenna 16 that operates in TM21 mode, antenna 16 having a field coupling feed 17 and a single mode separator 18 that is located at 22.5 degrees from feed 17 (see H. Hakano, K. Fujimori, J. Yamauchi, xe2x80x9cA LOW-PROFILE CONICAL BEAM LOOP ANTENNA WITH AN ELECTROMAGNETICALLY COUPLED FEED SYSTEM,xe2x80x9d IEEE Trans. On Ant. And Prog., Vol 48, No. 12, December 2000).
One problem in providing a low profile antenna is that of antenna bandwidth. Bandwidth typically is proportional to the distance between the antenna radiating/receiving element(s) and the antenna ground plane; i.e., the volume of the antenna (see Chu, L. j., xe2x80x9cPHYSICAL LIMITATIONS OF OMNI-DIRECTIONAL ANTENNASxe2x80x9d, J. Appl. Phys, Vol 19, December 1948, pp. 1163-1175). Hence, it is advantageous to provide that the radiating/receiving element (herein after radiating element) of a low profile antenna be at the greatest distance above the ground plane as is possible, while still satisfying the low profile requirement.
This invention provides a thin, disk-shaped, two antenna assembly for use in radiating and receiving both CP and linear electromagnetic signals of the type usually used in satellite communication systems.
In accordance with the invention, a CP ring antenna and a top-loaded monopole antenna occupy a common disk-shaped, or cylindrical-shaped, volume that has a generally flat bottom surface generally parallel to a flat top surface.
A ring-shaped radiating element of the ring antenna and the top loading disk of the monopole radiating element occupy a common plane at, or adjacent to, the generally top flat surface of this disk-shaped volume. That is, the radiating element of the ring antenna and the radiating disk of the monopole antenna may be generally coplanar.
The generally flat bottom surface of this disk-shaped volume includes a metal ground plane that may be carried by the bottom surface of a generally flat printed circuit board (PCB). In use, it is intended that antenna assemblies in accordance with the invention be physically oriented such that the ground plane is located in a generally horizontal plane.
The top-loaded monopole antenna (which may comprise two parallel and vertically extending metal posts) is located approximately concentric within the ring antenna in order to minimize electromagnetic coupling between the monopole antenna and the ring antenna. The top-loaded monopole antenna is physically supported by the PCB, and an air dielectric is associated with the monopole antenna.
Electronic components that are used by the monopole antenna and/or the ring antenna are located within a ring-shaped void that exists between a dielectric ring whose top surface supports the ring antenna. These electronic components may be mounted on the top surface of the ground plane at a location that is under the radiating ring of the ring antenna and under the top-loading disk of the monopole antenna.
The metal ring of the ring antenna may be in the form of meandering metal line that forms a circle, or it may be in the form of a wide or a narrow metal line that forms a circle. Metal perturbations or mode separators cooperate with this metal ring in order to preserve the symmetry of the ring antenna and in order to retain a symmetrical radiation pattern for the ring antenna.
At least one metal feed post is provided for the metal ring of the ring antenna and at least one generally centrally located metal post forms the monopole radiating element.